The term "epoxy resins" defines a well known class of organic compounds, each member of which bears a plurality of 1,2-epoxy groups. Such epoxy resins are commercially available and are used as adhesives, surface coatings, potting and encapsulating compositions, components in laminates, fillers and powder coatings, and the like. In such utilities the epoxy resins are normally formulated with one or more curing agents and cure rate accelerators just prior to use, thus giving compositions which are thermally curable at satisfactory reaction rates and/or at lower reaction temperatures.
Normally the epoxy resin compositions are marketed as a two-package system wherein one package contains the epoxy resin and the second package contains the curing agent and cure rate accelerator. This marketing approach has inherent technical problems which make it highly desirable to market a one-package system in which the epoxy resin, the curing agent, accelerator and other components are blended together under controlled conditions. Such a one-package system would eliminate costly field mixing by the consumer who in many instances does not have the proper equipment to institute a thorough blend of the components and/or a proper appreciation of the need to make and apply a homogeneous mixture of the components for optimum results.
The need for a one-package epoxy resin system is therefore widely recognized by the industry and is a goal sought by many manufacturers.
Various problems have been observed in quest of this goal, however, which have been the subject of much research. For example, most epoxy curing agents react readily with epoxy resins at conventional storage temperatures and are, therefore, obviously unsatisfactory. Those few epoxy curing agents which are latent (i.e., they do not react with the epoxy resins at conventional storage temperatures) often require high temperatures and/or long reaction times to achieve a satisfactory degree of cure. This problem has been solved in part by the inclusion of various accelerators to shorten the cure time and lower the reaction temperature. Unfortunately, such accelerators often tend to deleteriously affect the shelf-life stability of the formulated compositions and/or cause large voids and an undesirable appearance in the final product by virtue of the fact that their thermal degradation products are gases.
Some of these problems were apparently solved, in part at least, by using the accelerators described by Nawakowski et al. U.S. Pat. No. 3,386,956) and Simms et al. (U.S. Pat. No. 3,759,914). The accelerators there described were aromatic ureas having an aromatic hydrocarbon nucleus bearing one or more urea substituents and, optionally, certain other substituents in addition to the urea substituent(s). The specific compounds named normally had the ring substituents located on the .beta. or .gamma. carbon atom of the aromatic nucleus, relative to the carbon atom bearing a urea substituent. The manner(s) of making and using such accelerators in combination with latent amine curing agents in convention epoxy resin compositions are likewise taught in the above patents. The disclosure of U.S. Pat. Nos. 3,386,956 and 3,759,914 are incorporated herein by reference.